1. Field of the Invention
The present invention relates to monitoring contamination on a surface and more particularly to the use of second-order nonlinear optics to determine the level of contamination with a high degree of specificity.
2. Description of the Related Art
In nonlinear optical, or so-called wave mixing processes, outputs are produced at sum, difference or harmonic frequencies of the input(s). Second order nonlinear optics or three-wave mixing involves combining two inputs to produce one output at one of these combination frequencies. Using second order nonlinear optical surface spectroscopy to examine the physical properties and behavior of a surface or interface was originally proposed in the 1960's, in “Light Waves at the Boundary of Nonlinear Media” by Bloembergen and P. S. Pershan, The Physical Review, 128, Page 193 (1962). Experimental work involving second harmonic generation was also performed. The authors of the publication realized that, under certain conditions, second order nonlinear optical effects would be constrained to a surface. However, because lasers at the time were comparatively feeble, impractical, slow, etc., there was little subsequent work done on the development of second harmonic generation or, more generally, second order nonlinear optical (NLO) processes at surfaces until considerably later, when lasers had reached a certain level of technical maturity.
Recently, researchers have reviewed NLO processing and concluded that lasers had developed enough that they could be used for studying the physical and chemical properties of surfaces and interfaces. For example, a theoretical study of the physics of the interface, and not its engineering aspects, has been performed. See Journal of Vacuum Science and Technology B, Volume 3, Number 5, September October 1985, Pages 1464-1466, Y. R. Shen, “Surface Studies by Optical Second Harmonic Generation: an Overview.” The physical properties which make second order nonlinear optics useful for surface sensitive characterizations are discussed in this publication.
In U.S. Pat. No. 5,294,289, T. F. Heinz et al. discuss the use of second harmonic generation as a means to monitor the epitaxial growth of silicon semiconductor structures in a high vacuum chamber. Specifically, they examined the spectroscopic response at the interface between the electronically active silicon and the insulative layer of calcium fluoride. Changing the wavelength of the input induced a resonant enhancement of the intensity of the surface second harmonic signal. By monitoring the magnitude of the resonance, they could ascertain whether the insulator was present on the surface and whether it had electronically binded to the underlying semiconductor. The system that is used examines the total intensity only of the second harmonic light that is generated and there is no discussion of the use of optical difference-frequency generation between two input sources. There is also no discussion of contamination detection.
In U.S. Pat. No. 5,623,341, J. H. Hunt discusses the use of sum-frequency generation for the detection of contamination and corrosion on engine parts. In this incarnation, one of the inputs is a tunable IR beam that is tuned to a resonance of the contamination or corrosion on the surface. The efficiency of the sum-frequency process is increased (so-called resonant enhancement) when the IR beam is resonant with a contaminant. If the contaminant is not present, there is no resonant enhancement. By comparing on and off resonant signals, the presence and level of contaminant can be deduced. However, there is no discussion of applying difference-frequency generation to observe a material resonance.
In U.S. Pat. No. 5,875,029, P. C. Jann et al. describe a versatile optical inspection instrument and method to inspect magnetic disk surfaces for surface defects. The device provides surface position information of the defects. However, the technique involves only linear optical processes. That is, the input and output light wavelengths are the same. There is also no discussion of contamination.
In U.S. Pat. No. 5,883,714, Jann et al. describe a versatile optical inspection instrument and method to inspect magnetic disk surfaces for surface defects. The device is based on interferometric measurement and detects contaminants by measuring the Doppler shift in the light that results from scanning the light onto a contaminant or defect. By scanning, the device provides surface position information of the defects. However, the technique involves only linear optical processes and senses only phase changes. That is, the input and output light wavelengths are the same and there is no discussion of contamination.
In U.S. Pat. No. 5,898,499, J. L. Pressesky discusses a system for detecting local surface discontinuities in magnetic storage discs. The device is an interferometric detector which scans the disc in a spiral motion. Local defects cause local changes in phase which are measured by interferometric techniques. This is a linear optical technique.
In U.S. Pat. No. 5,932,423, T. Sawatari et al. discuss a scatterometer for detecting surface defects in semiconductor wafers. This device is a linear interferometric device.
In U.S. Pat. No. 5,973,778, J. H. Hunt discusses the use of second harmonic generation for investigating molecular alignment within a thin polyimide film. The technique uses changes in the second harmonic polarization to determine surface molecular alignment. There is no discussion of difference-frequency generation from the surface to be interrogated. Furthermore, there is no discussion of semiconductor materials or contamination. The nonlinear optical response of a semiconductor will be quite different than that of a liquid crystal film.
In U.S. Pat. No. 6,317,514 B1, S. Reinhorn et al. discuss a method and apparatus for inspecting a wafer surface to detect the presence of conductive material on the wafer. The device uses UV initiated electron emission to determine the location of conductive areas. Those areas which are metal will emit electrons. If the area, which is supposed to be conductive, is not, there will be no electron emission.
In U.S. Pat. No. 6,359,451 B1, G. N. Wallmark discusses a system for testing for opens and shorts between conductor traces on a circuit board. The technique uses electron scattering to perform its diagnostics and has no optics associated with it.